The regulation of gene expression during the yeast cell cycle will be examined for two classes of yeast genes: (1) the cell division cycle genes (cdc genes) which code for gene products essential for DNA replication, (2) the histone genes, which code for a group of nuclear proteins that are periodically synthesized during DNA replication. The patterns of cdc and histone gene expression will first be established for the normal cell cycle and then examined in mutants which are blocked at specific stages of the cell cycle. Gene expression will be monitored by following the synthesis of the cdc and histone gene products either by 2D polyacrylamide gel electrophoresis of total yeast proteins or in vitro translation of purified mRNAs. mRNAs will be isolated by hybridization to histone or cdc genes carried on bacterial plasmids. Hybrid plasmids carrying two yeast histone genes have already been identified and characterized, and recombinant plasmids harboring cdc genes will be isolated and identified by the technique of yeast transformation.